Resistance to sterol-demethylation inhibiting fungicides in populations of Venturia inaequalis from Pennsylvania apple orchards
Open Access
- Author:
- Pfeufer, Emily E.
- Graduate Program:
- Plant Pathology
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 01, 2010
- Committee Members:
- Henry Ngugi, Thesis Advisor/Co-Advisor
Henry Ngugi, Thesis Advisor/Co-Advisor - Keywords:
- Venturia inaequalis
apple scab
fungicide resistance - Abstract:
- Apple scab, caused by Venturia inaequalis, is the most economically important fungal disease of apple in the eastern United States. Over the past 25 years, apple growers have relied on sterol demethylation inhibiting (DMI) fungicides for scab control, but reduced efficacy has recently been noted. The aim of this study was to evaluate the sensitivity of V. inaequalis isolates from Pennsylvania to DMI fungicides. In 2008 and 2009, leaves and immature fruit with scab symptoms were collected from 20 commercial orchards. Growers provided management history of the sampled plots. A total of 479 single-spore cultures were isolated from the tissues and maintained individually. Each isolate was tested for sensitivity to DMI fungicides on 1/4-strength PDA plates amended with a range of concentrations of myclobutanil, fenbuconazole, or difenoconazole. Mean effective concentration for 50% inhibition (EC50) values of each fungicide were calculated for populations from 14 orchards. Relative growth (RG) values were calculated and isolates with RG > 75 on plates amended with 0.5 g/mL, were scored as resistant to the particular fungicide. Widespread shifts toward resistance to the three DMI fungicides were noted with EC50 values of 2.139  0.090 (mean  standard error), 0.839  0.053 and 0.158  0.008, for myclobutanil, fenbuconazole, and difenoconazole, respectively. Based on a 0.5 g/mL threshold, 64% of isolates were resistant to myclobutanil, and 24% were resistant to fenbuconazole. Less than 1% of the isolates were cross-resistant to all three fungicides, but 22% were cross-resistant to myclobutanil and fenbuconazole. Failure to use dormant copper sprays, older trees, larger orchards, orchards comprised of ten or more cultivars, mixed-cultivar orchards, and isolates from the ‘York Imperial’ cultivar were positively correlated (0.0001 < P < 0.05) with the incidence of resistant isolates. Isolates from orchards that were not treated with copper were twice as likely to be resistant to myclobutanil (odds ratio = 2.015; P = 0.0009). Growers can focus on using management practices, particularly dormant copper sprays, to reduce their risk of developing resistance to DMI fungicides.